Summary of BS 476: Part 20: 1987 Method for determination of the fire resistance elements of c***truction (general principle)
Purpose
The objective of determining fire resistance, as described in this Part and the associated Parts of BS 476, is to assess the beh***iour of a specimen of an element of building c***truction when subjected to defined heating and pressure conditi***. The method provides a means of quantifying the ability of an element to withstand exposure to high temperatures, by setting criteria by which the loadbearing capacity, the fire containment (integrity) and the thermal tran***ittance (insulation) functi*** can be adjudges.
BS 476: Part 20 provides guidance on the general principles of the test method for fire resistance and should be used in conjunction with BS 476: Part 21 to 23.
Test specimen
The test specimen and any associated c***truction forming a test c***truction are identical to, or representative of, the element of building c***truction that is to be evaluated.
For non-separating elements, separating elements that are only required to resist fire from one side in use, and for vertical symmetrical separating elements, only one specimen is tested. For vertical asymmetrical separating elements that are required to resists fire from either side, the element is tested from each side using a separate specimen.
Wherever possible, test specimens are full-sized elements of building c***truction and unless it is impossible, the individual components making up a test specimen are also full-sized. Where the above is not possible, the minimum size of the elements shall be:
Non-separating elements:
Vertical: 3 m high
Horizontal: 4 m high
Separating elements:
Vertical: 3 m high by 3 m wide
Horizontal: 4 m span by 3 m wide
For non-loadbearing elements, the test c***truction is mounted as in practice. For loadbearing elements, the test loading is selected to represent the condition applicable to the element in practice.
Apparatus
This c***ists essentially of:
1. A furnace of a design appropriate to the type of specimen being tested to allow:
a. vertical separating elements to be exposed to the heating and pressure conditi*** on one face.
b. Horizontal separating elements to be exposed to heating and pressure conditi*** from the undersides.
c. Beams normally to be exposed to the heating and pressure conditi*** on three faces, i.e. the soffit and two sides, but provision should be made to enable beams to be tested with all four faces exposed.
d. Free standing columns to be exposed to the heating and pressure conditi*** equally on all four faces. The furnace is fuelled by either natural gas or LP Gas and has thermocouples linked to associated temperature measuring devices for determining the temperature at specific positi***. A pressure sensing probe is also sited within the furnace.
2. Loading equipment. A test loading is applied to the specimen either by dead weights or by a hydraulic or mechanical jacking system, or by a combination of both.
3. Specimen support systems to apply restraint at the edges or ends of the specimen.
4. Monitoring equipment c***isting if fixed specimen surface thermocouples, a roving surface thermocouple, a cotton pad for monitoring permeability, gap gauges for monitoring impermeability, deformation measuring devices and a radiometer.
Method
A representative sample of the element is exposed to a specified regime of heating under positve pressure and the performance of the test sample is monitored on the basis of criteria described in the Standard. Fire resistance of the test element is expressed as the time for which the appropriate criteria h***e been satisfied. The times so obtained are a measure of the adequacy of the c***truction in a fire but h***e no direct relati***hip with the duration of a real fire.
Performance criteria
The fire resistance of the test c***truction is assessed against one or more of the criteria for loadbearing capacity, integrity and insulation, whichever are relevant to the elements used in practice.
1. Loadbearing capacity – failure is deemed to h***e occurred when the specimen fails to support the test loading.
2. Integrity – failure is deem to occur when collapse or sustained flaming on the unexposed face occurs, or the criteria given for impermeability are exceeded.
3. Impermeability – failure has occurred when flames and or hot gases cause flaming or glowing of a cotton pad, or a 6 mm diameter gap is formed with a length greater than 150 mm or a 25 mm diameter gap is formed.
4. Insulation – failure has occurred if:
a. the mean unexposed face temperature increase by more than 140°C above its initial value, or
b. the temperature recorded at any position on the unexposed face is more than 180°C above the initial mean temperature.
The standardized temperature / time conditi*** used I this evaluation are representative of only one possible fire exposure condition at the fully developed fire stage and the method does not quantify the beh***iour of an element, for a precise period of time, in a real fire situation. In addition, due to restricti*** of size and the absence of surrounding c***truction, the laboratory test cannot reproduce the actual beh***iour pattern of an element in a fire. However, test data can provide a basis for making engineering evaluati***.